The present invention relates to a method of making a specimen for observing a specific portion of a semiconductor device through use of a transparent electron microscope and, more particularly, to a method of and an apparatus for making a specimen by milling with a focused ion beam.
Recently, focused ion beam (hereafter referred to as "FIB") milling has been used for making specimens of specific portions of semiconductor devices for use on a transparent electron microscope thereafter referred to as "TEM") in analyses of a gate portion of a specific memory cell and an interface of a metal contact of a specific contact hole of a semiconductor device. An example of such a method will be described below with reference to the drawings. As shown, for example, in FIG. 2, a surface of a specimen 42 is polished by a polishing apparatus to form a protruding part 1 of 30 to 100 .mu.m (typically, 50 .mu.m) in width and 10 to 100 .mu.m (typically, 50 .mu.m) in height so that a portion to be observed is positioned at the center of this protruding part.
As shown in FIG. 3, portions of both sides of the protruding part 1 are removed to a depth of d=3 to 10 .mu.m and a width of w=4 to 15 .mu.m using a focused ion beam 2 so as to leave a thin film part 3 having a desired thickness t at the center of the protruding part. The protruding part is milled so that a portion to be observed is formed in this thin film part 3. The thickness t of this thin film part 3 needs to be approximately 100 nm or less to carry out a TEM observation. To leave such a small thickness, the protruding part is processed to leave a film having a thickness of 1 .mu.m by using a focused ion beam of approximately 0.5 to 1 .mu.m in beam diameter in an initial stage of processing, and the thin film part 3 with thickness t is further gradually milled by using a thinner beam of approximately 0.1 .mu.m or less in beam diameter to finish a specimen for TEM observation, which is provided with a final thickness of approximately 100 nm or less.
A first prior method related to the above-described type of processing, for example, is described in Japanese Patent Application Disclosure Hei 5-15981, which discloses a method of milling a specimen for use in SEM observation of a cross section, which method is adapted to mill a mark so as to make it possible to control a position of a cross section to be finally obtained and is adapted to set a finishing position by using this scanning ion microscope image (SIM image).
As a second prior method, the "J. Vac. Sci. Technol. Bll, (3) (May/June), pp 531 to 535, 1993" published by the U.S. Society of Vacuum, discloses a method of milling a specimen for use in TEM observation by which an electron beam is irradiated on the specimen during FIB processing and the thickness of a film which is formed by milling is obtained by observing secondary electrons or reflecting electrons generated therefrom.
In most cases, the positional drift of the focused ion beam 2 is approximately 0.1 .mu.m to 0.5 .mu.m/10 minutes. Therefore, if the laser beam drifts to the center of the thin film part 3, as shown in a plan view of a relevant part of a milled surface of the specimen in FIG. 4, the thin film part 3 is often excessively milled and the portion to be observed is inadvertently milled off.
When the thickness t of the thin film part 3 approaches 200 nm, it becomes difficult to identify the thin film part 3 from a scanning ion image and to set a milling area 5, and therefore there is a risk that the portion to be observed may be inadvertently milled away and damaged. In the case of the first prior method, marks 6a and 6b are provided in an area shown as an observing area 4 of the thin film part 3 in FIG. 4, and a milling area 5 can be determined by observing scanning ion microscope images (SIM image) of these marks. However, there has been a risk that, when the thickness of the thin film part 3 approaches approximately 200 nm, the top part of the thin film part 3 may be milled while the scanning ion image is observed and the thin film part 3 may be excessively thinned.
In the case of the second prior method, high costs are required to provide an electron gun for the SEM, a power supply and a controller, and it is spatially difficult to simultaneously reduce sufficiently the operating distance of the focused ion beam optics and that of the electron beam optics, since the electron gun is arranged nearby the workpiece. Therefore, installation of the SEM on a practical focused ion beam milling unit includes two problems, that is, the high price of the apparatus and the difficulty in focusing the laser beam to obtain sufficiently thin beam.
In the above prior method, practical means for observing the thickness distribution of the thin film part and means for detecting an electron beam which passes through the thin film part are employed. For this reason, the prior methods include a problem in that, if a specimen for TEM observation is made by using an ordinary focused ion beam milling machine, a failure may be often repeated, the work efficiency is extremely low and a lot of time is required to obtain the data for TEM observation. In addition, there is another problem in that, if only one specimen is available, it cannot be milled to be sufficiently thin due to a fear of probable breakage.